This invention relates generally to processes for the production of metal-oxide varistors and more particularly, zinc-oxide-based varistors incorporating at least one metal oxide as a dopant. The invention is a result of a contract with the United States Department of Energy.
Zinc-oxide-based varistors are ceramics whose highly nonlinear electrical-conduction characteristics make them suitable for use as surge arrestors or voltage limitors in electrical systems. Non-ohmic behavior in zinc oxide (ZnO) is achieved by doping with one or more oxides, such as those of bismuth, antimony, cobalt, and rare earths. Doping results in the formation of voltage barriers at the grain boundaries. The increase in the varistor conductivity is related to temporary breakdown of the grain-boundary barriers; thus, the varistor breakdown voltage (V.sub.b) is inversely related to the average ZnO grain size.
Commercial processes for the production of ZnO-based varistors involve sintering mixtures of the ZnO and dopant powders in air or vacuum at 1200.degree. to 1400.degree. C. The typical ZnO-based varistor includes Bi.sub.2 O.sub.3 as a dopant. Unfortunately, the ZnO--Bi.sub.2 O.sub.3 system has a eutectic at 750.degree. C., with the result that appreciable liquid formation and grain growth occur during sintering. Consequently, varistors designed for high voltage applications (e.g., lightning arrestors for electrical-transmission systems at 10.sup.6 volts) are undesirably large.
The previous art includes the following article, describing an experiment conducted to demonstrate the desirability of using a flatplate heater press to produce ceramic slugs with a large aspect ratio: "Hot Press With Flat Plate Heaters and Its Application To The Fabrication Of Large Varistor Slugs", American Ceramic Society Bulletin, Vol. 59, No. 5, May 1980. In that experiment, densification was accomplished in an oxygen-containing atmosphere. That is, a cold-pressed compact was mounted in a hot-pressing system; after evacuation of the system, a fixed, low load was applied to the compact while the temperature was gradually increased from ambient. The system then was back-filled with oxygen, following which the temperature was raised appreciably and the load was increased by a factor of about 4.5 to effect densification. The following article describes the fabrication of ZnO-based varistors by a single-step hot-pressing process conducted in air: "Characterization of High Field Varistors in the System ZnO--CoO--PbO--Bi.sub.2 O.sub.3, " American Ceramic Society Bulletin, Vol. 59, No. 6, June 1980.